Roll compaction to produce strip currently is applied to powders of a range of metals and their alloys. These metals include steel, stainless steel, iron-silicon, cobalt-iron, copper, nickel, chromium, aluminium and titanium. Current roll compaction involves consolidation of metal powder, which may be elemental, blended elemental (BE) or pre-alloyed (PA) powder, by a standard rolling mill to produce a “green” strip. By a batch or continuous operation, the green strip undergoes further sintering and re-rolling, to produce a flat strip product with a tailored degree of porosity or fully dense sheet.
Direct powder rolling technology has a number of advantages over the conventional ingot/wrought processing route to sheet production. These advantages include:                (a) lower operating costs and also lower capital equipment requirements by minimising the number of processing steps;        (b) production of high purity sheet with minimal risk of segregation and at a higher yield;        (c) facilitating production of fine-grained, high strength strip exhibiting a lower effect of rolling orientation on mechanical properties and grain texture; and        (d) facilitating production of specialty materials difficult to produce by more conventional means, such as strip which is bimetallic, porous, composite bearing, functionally graded and/or clad, as well as strip of those alloys that are not readily amenable to hot and/or cold working.        
There are three powder processing routes that have most widely been used. These differ in the preparation of the green strip. In the first route, the powder is mixed with a binder prior to the powder/binder mix being subjected to roll compaction. In the second and third routes, dry powder without binder is subjected to roll compaction, at ambient or an elevated temperature, respectively. With each of the three routes, the green strip is sequentially sintered for an extended period to a high density, and then subjected to hot and/or cold rolling. After hot rolling the green strip, the resultant densified strip may be cold rolled prior to being annealed or annealed prior to being cold rolled. After initial cold rolling of the densified strip, the resultant cold rolled strip may be subjected to further sintering and cold rolling, prior to being annealed.
The use of a binder, as in the first of those routes, is not desirable as it results in the end product metal strip containing inclusions which diminish physical properties. Thus, the second and third routes have been preferred for the production of strip of various metal powders, including titanium and titanium alloy strip. The procedures of these routes are illustrated by British patent specifications GB 2107738A and GB 2112021A, both by Imperial Clevite Inc, U.S. Pat. No. 4,594,217 to Samal, U.S. Pat. No. 4,917,858 to Eylon et al, and US patent publication US 2006/0147333A1 by Moxson et al.
The process of GB 2107738A involves passing a powder mixture of an enriched metal alloy and a filler metal through a powder rolling mill to produce a densified mass having a density of at least 80% theoretical, and sintering the densified mass to cause interparticle bonding and diffusion to produce a homogeneous mass. The filler metal may be titanium or a titanium alloy, while the alloy may contain aluminium, zinc, magnesium and copper. The process of GB 2112021A differs from that of GB 2107738A principally in that the initially formed densified mass can have a density as low as 50% of the theoretical density, and it is cold rolled prior to sintering.
U.S. Pat. No. 4,594,217 relates to direct powder rolling of dispersion strengthened copper, iron, nickel or silver and its process is relevant to titanium only in that titanium oxide is one of various refractory oxides that may be used to achieve dispersion strengthening. The powder rolling is to produce green strip with a density of from 90% to 95% of theoretical, and the green strip is sintered in an inert atmosphere and for a period of time to cause the particles to adhere and form a solid body which then is subjected to at least one cycle of cold rolling and re-sintering.
U.S. Pat. No. 4,917,858 is specific to production of titanium aluminide foil, of either Ti3Al or TiAl. Blended elemental powders, which may contain minor alloying additions, are rolled to produce green foil, after which the foil is sintered, such as to a density of from 88% to 98% of the theoretical density, and then subjected to a suitable form of hot pressing, such as by vacuum hot pressing, hot isostatic pressing, hot rolling or hot die forging.
US patent publication US 2006/0147333 relates to a process for the production of sheet, and other flat products, of titanium. In this, a green strip is produced by passing powder through a first set of unequally sized rolls, and then through a second set of larger rolls. The strip from the first set of rolls is to achieve a density of 40 to 80% of the theoretical density and, due to the rolls of that set being unequally sized, the strip is bent so as to pass to the second set. The rolls of one of the two sets are rotated relative to each other to achieve densification by shear deformation. The strip from the second set of rolls is subjected to multiple stages of cold re-rolling, said to achieve about 100% of the theoretical density, after which the strip is sintered under vacuum or a protective atmosphere. The powder mix used is a mix of CP titanium matrix powder and an alloying powder having a particle size at least ten times smaller than the matrix powder, to produce, for example, fully dense Ti-6Al-4V alloy.
While titanium strip can be produced by processes such as detailed above, there remains a problem which also applies to titanium strip produced by the ingot/wrought processing route. This arises with that cost component, of the overall cost of producing the sheet, attributable to the production of titanium metal, whether as powder or ingots, respectively. Relative to the production of strip of other metals, the metal production cost component for titanium strip is very high. Thus, until a more cost efficient process is developed for the production of titanium metal, it is necessary to seek cost reducing efficiencies at all production stages in order to increase the competitiveness of titanium strip with respect to strip of other metals.
The present invention seeks to provide an alternative process for the production of titanium flat product, such as strip or plate, which involves densification of green flat material of titanium powder and which, at least in some forms, enables more cost effective production.